The invention is applicable to the field of static conversion where rectification and filtering of incoming AC power is intended to provide a smooth DC output. This is particularly the case in AC motor drives where the DC link voltage that must be filtered exceeds the rating of the electrolytic capacitors connected in series thereacross. The solution has been to connect in series several capacitors in order to achieve the required voltage capability. A problem arises in this situation. Since the capacitors are neither perfect nor identical, they have a leakage current that can widely vary from one capacitor to the next. Such difference in leakage current causes the capacitor having the lesser leakage to be charged to a voltage which can be unacceptable under the higher leakages of the other capacitors forming a series string therewith. The standard practice to cope with this unbalanced leakage problem has been to add resistors in parallel, one with each capacitor, or set of parallel capacitors mounted in series string, so as to absorb the leakage currents and maintain a voltage balance within a reasonable range. The value of such leakage current sharing resistors is determined from a consideration of the range of leakage current for each capacitor, the range of DC link voltage and the maximum allowable voltage unbalance.
Voltage balancing resistors in parallel to each capacitor of the string are undesirable for several reasons, among which cost and power dissipation are the main drawbacks. In addition, a hermetically closed housing for dust-tight application of an adjustable frequency AC motor drive becomes unacceptable because of the heat build-up with such resistors. An alternative is to minimize the heat loss inside the enclosure by choosing externally mounted long lead time single source resistors, an expensive solution.